In an automobile or a railway vehicle system, it is necessary to accurately grasp a state of charge of a battery during vehicle operation in a power storage system that performs power supply to a driving motor during the vehicle operation and electric energy collection during deceleration or stop.
In the past, as a method of estimating a state of charge, a method of calculating a state of charge from an integrated value of charge and discharge current values and a method of estimating a state of charge from an open circuit voltage of a battery are known. In a related art in which both the methods are used, a method of estimating a state of charge from current integration when a battery is charged and discharged and estimating a state of charge by estimating an open circuit voltage during standby is adopted (e.g., Patent Literature 1 described below). A state of charge can be estimated from an open circuit voltage. However, a technology for predicting an open circuit voltage in a closed circuit that performs charge and discharge is necessary. Concerning the technology, there is a method of calculating internal resistance from measurement values of an energization current and a battery voltage during the closed circuit using an equivalent circuit model of a battery (e.g., Patent Literatures 1, 2, and 3 described below).